Power scheme for implant stimulators on the human or animal body

ABSTRACT

A power scheme for an implant on a human or animal body comprises: a charging circuit to provide power to deliver controlled stimulation currents to a body tissue; a capacitive storage arrangement connected with the charging circuit and charged by the charging circuit; a shunting arrangement to limit voltage on the capacitive storage arrangement; a driver array configured to transfer charges from the capacitive storage arrangement to the tissue; and an electrode array connected with the driver array and the tissue.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of, and claims priority to,U.S. application Ser. No. 11/598,965, filed Nov. 14, 2006, for PowerScheme for Implant Stimulators on the Human or Animal Body.

FEDERALLY SPONSORED RESEARCH

This invention was made with government support under grant No.R24EY12893-01, awarded by the National Institutes of Health. The U.S.Government may have certain rights in the invention.

BACKGROUND

1. Field

The present disclosure relates to implants for humans or animals. Inparticular, it relates to a power scheme for implant stimulators (alsoreferred to as implants in the present application) on the human oranimal body.

2. Related Art

The following paragraphs will introduce some art possibly related to thepresent application.

In 1755 LeRoy passed the discharge of a Leyden jar through the orbit ofa man who was blind from cataract and the patient saw “flames passingrapidly downwards.” Ever since, there has been a fascination withelectrically elicited visual perception. The general concept ofelectrical stimulation of retinal cells to produce these flashes oflight or phosphenes has been known for quite some time. Based on thesegeneral principles, some early attempts at devising a prosthesis foraiding the visually impaired have included attaching electrodes to thehead or eyelids of patients. While some of these early attempts met withsome limited success, these early prosthetic devices were large, bulkyand could not produce adequate simulated vision to truly aid thevisually impaired.

In the early 1930's, Foerster investigated the effect of electricallystimulating the exposed occipital pole of one cerebral hemisphere. Hefound that, when a point at the extreme occipital pole was stimulated,the patient perceived a small spot of light directly in front andmotionless (a phosphene). Subsequently, Brindley and Lewin (1968)thoroughly studied electrical stimulation of the human occipital(visual) cortex. By varying the stimulation parameters, theseinvestigators described in detail the location of the phosphenesproduced relative to the specific region of the occipital cortexstimulated. These experiments demonstrated: (1) the consistent shape andposition of phosphenes; (2) that increased stimulation pulse durationmade phosphenes brighter; and (3) that there was no detectableinteraction between neighboring electrodes which were as close as 2.4 mmapart.

As intraocular surgical techniques have advanced, it has become possibleto apply stimulation on small groups and even on individual retinalcells to generate focused phosphenes through devices implanted withinthe eye itself. This has sparked renewed interest in developing methodsand apparati to aid the visually impaired. Specifically, great efforthas been expended in the area of intraocular retinal prosthesis devicesin an effort to restore vision in cases where blindness is caused byphotoreceptor degenerative retinal diseases such as retinitis pigmentosaand age related macular degeneration which affect millions of peopleworldwide.

Neural tissue can be artificially stimulated and activated by prostheticdevices that pass pulses of electrical current through electrodes onsuch a device. The passage of current causes changes in electricalpotentials across retinal neuronal cell membranes, which can initiateretinal neuronal action potentials, which are the means of informationtransfer in the nervous system.

Based on this mechanism, it is possible to input information into thenervous system by coding the sensory information as a sequence ofelectrical pulses which are relayed to the nervous system via theprosthetic device. In this way, it is possible to provide artificialsensations including vision.

Some forms of blindness involve selective loss of the light sensitivetransducers of the retina. Other retinal neurons remain viable, however,and may be activated in the manner described above by placement of aprosthetic electrode device on the inner (toward the vitreous) retinalsurface (epiretinal). This placement must be mechanically stable,minimize the distance between the device electrodes and the retinalneurons, and avoid undue compression of the retinal neurons.

In 1986, Bullara (U.S. Pat. No. 4,573,481) patented an electrodeassembly for surgical implantation on a nerve. The matrix was siliconewith embedded iridium electrodes. The assembly fit around a nerve tostimulate it.

Dawson and Radtke stimulated a cat's retina by direct electricalstimulation of the retinal ganglion cell layer. These experimentersplaced nine and then fourteen electrodes upon the inner retinal layer(i.e., primarily the ganglion cell layer) of two cats. Their experimentssuggested that electrical stimulation of the retina with 30 to 100 uAcurrent resulted in visual cortical responses. These experiments werecarried out with needle-shaped electrodes that penetrated the surface ofthe retina (see also U.S. Pat. No. 4,628,933 to Michelson).

The Michelson '933 apparatus includes an array of photosensitive deviceson its surface that are connected to a plurality of electrodespositioned on the opposite surface of the device to stimulate theretina. These electrodes are disposed to form an array similar to a “bedof nails” having conductors which impinge directly on the retina tostimulate the retinal cells. U.S. Pat. No. 4,837,049 to Byers describesspike electrodes for neural stimulation. Each spike electrode piercesneural tissue for better electrical contact. U.S. Pat. No. 5,215,088 toNorman describes an array of spike electrodes for cortical stimulation.Each spike pierces cortical tissue for better electrical contact.

The art of implanting an intraocular prosthetic device to electricallystimulate the retina was advanced with the introduction of retinal tacksin retinal surgery. De Juan, et al. at Duke University Eye Centerinserted retinal tacks into retinas in an effort to reattach retinasthat had detached from the underlying choroid, which is the source ofblood supply for the outer retina and thus the photoreceptors. See,e.g., E. de Juan, et al., 99 Am. J. Opthalmol. 272 (1985). These retinaltacks have proved to be biocompatible and remain embedded in the retina,and choroid/sclera, effectively pinning the retina against the choroidand the posterior aspects of the globe. Retinal tacks are one way toattach a retinal electrode array to the retina. U.S. Pat. No. 5,109,844to de Juan describes a flat electrode array placed against the retinafor visual stimulation. U.S. Pat. No. 5,935,155 to Humayun describes aretinal prosthesis for use with the flat retinal array described in deJuan.

Retinal implants receiving power from an external unit through aninductive power link coupled through coils are known. When the coilsizes and positioning are limited by the physical conditions, the powerdelivering efficiency can be reduced dramatically, in which case themaximum power to the implant may be limited. On the other hand, a higheramount of power used by the implant also means a worse condition interms of thermal dissipation.

Both of the situations above require a reduced power demand by theimplant stimulator. For applications that need a large number ofstimulation channels, such as in the case retinal prosthesis, theefficiency between the output current and the input power becomescritical.

SUMMARY

According to a first aspect of the present disclosure, a power controlsystem for an implant on a human or animal body is shown, comprising: acharging circuit to provide power to deliver controlled stimulationcurrents to a tissue of the human or animal body; a capacitive storagearrangement connected with the charging circuit and charged by thecharging circuit; a shunting arrangement to limit voltage on thecapacitive storage arrangement; a driver array configured to transfercharges from the capacitive storage arrangement to the tissue; and anelectrode array connected with the driver array and the tissue.

According to a second aspect of the present disclosure, a shuntingcircuit to regulate capacitor voltage in an implant for a human oranimal body is shown, the implant comprising a capacitive storagearrangement, the shunting circuit comprising: a current sensorcomprising terminals connected in parallel with the capacitive storagearrangement, the current sensor sinking current from the capacitivestorage arrangement when the voltage of the capacitive storagearrangement reaches a voltage value.

According to a third aspect of the present disclosure, a constantcurrent type electrode driver for an implant for a human or animal bodyis shown, the electrode driver comprising: a driver controller togenerate anodic and cathodic stimulation switching signals and generatean output current defining a current amplitude for stimulation; aconversion circuit to convert the output current into an anodic currentor a cathodic current; and a switching arrangement to allow selectionbetween the anodic current or the cathodic current.

According to a fourth aspect of the present disclosure, a compliancemonitoring circuit to monitor and control compliance voltage of anelectrode contacting a tissue of a human or animal body is shown, theelectrode being connected with an electrode driver comprising an outputMOSFET transistor having a drain-source voltage Vds, a gate-sourcevoltage Vgs and a threshold voltage Vt, the compliance monitoringcircuit monitoring a condition Vgs−Vds>Vt and generates an alert signalwhen such condition is met.

According to a fifth aspect of the present disclosure, a power monitorto monitor charging and draining conditions of i) a capacitive storagearrangement and ii) shunting circuits comprised in an implant for ahuman or animal body is shown, the power monitor comprising: a currentmonitor, the current monitor comprising an analog to digital converterdigitizing analog inputs from the shunting circuits and outputting anoutput current level; and a capacitor monitoring circuit, to monitorwhether implant power falls below a power value.

According to a sixth aspect of the present disclosure, a method tocontrol power in an implant for a human or animal body is shown, theimplant comprising electrodes contacting a tissue of the body, themethod comprising: capacitively storing electric charges; providing theelectric charges to the electrodes; monitoring when the electric chargesare above a high value or below a low value; and controlling theelectric charges when above the high value or below the low value.

According to a seventh aspect of the present disclosure, a circuit isshown, comprising: an electrode driver array; storage capacitorsconnected with the electrode driver array, wherein charge stored on thestorage capacitors is adapted to be transferred to an array ofelectrodes connected with the electrode driver array; a charging circuitcharging the storage capacitors; and a monitoring circuit tocontinuously monitor voltage on the storage capacitors and to control atleast one between the electrode driver array and the charging circuitbased on the voltage.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1-1, 1-2 and 1-3, to be seen as connected side by side, show ageneral diagram of the implant power control scheme in accordance withthe present disclosure.

FIG. 2 shows a circuital scheme of an electrode driver.

FIG. 3 is a timing diagram showing current and voltage signals relatedto the present disclosure.

FIG. 4 shows a circuital scheme of one of the shunt regulator circuitsof FIG. 1.

FIG. 5 shows a circuital scheme of the compliance monitor circuit ofFIG. 1.

FIG. 6 is a timing diagram showing signals related to the circuit ofFIG. 5.

FIG. 7 shows a circuital scheme for the power monitor circuit of FIG. 1.

FIGS. 8-1 and 8-2, to be seen as connected side by side, are a flowchart showing the power control flow of the scheme shown in FIG. 1.

DETAILED DESCRIPTION

FIGS. 1-1, 1-2 and 1-3 are a diagram of an implant power control schemein accordance with an embodiment of the present disclosure.

A retinal implant 10 receives power from an external unit 20 through aninductive power link 30 coupled through coils. Control and statusinformation are exchanged between retinal implant 10 and external unit20 through data link 40. While the embodiment of FIGS. 1-1, 1-2, 1-3 andthe following figures is concerned with a retinal implant, the personskilled in the art will appreciate that the same scheme can be used alsofor other types of implants on the human or animal body, such ascochlear implants or implants to restore neuronal functions impaired dueto injuries or diseases.

Power at the implant side is received by implant coil 50. Implant coil50 is tunable with capacitor C3 to the power carrier frequency. Thereceived AC power is converted into DC power by a rectifier circuit 60.Rectifier circuits are known per se to the person skilled in the art. Inthe case at issue, the rectifier circuit 60 can comprise, by way ofexample, diode bridges or MOSFET circuits.

The output V+, VM, V− of rectifier circuit 60 provides the power todeliver controlled stimulation currents to the tissue 70. A smallportion of the output of rectifier 60 can be tapped out or diverted tosupply circuits for other operations through a regulator circuit 75.Such operations can include RF data receiving and transmitting, logiccontrol, signal measurements and so on.

The output of rectifier circuit 60 continuously charges two capacitivestorage arrangements which are shown as C1 and C2 in FIG. 1-2 providedto supply all electrode drivers for bi-phase or multi-phase stimulation.Those capacitive arrangements could also be represented as arrays ofcapacitors or storage devices based on a capacitive behavior to boostoutput power.

The voltages of C1 and C2 are limited by shunt regulator circuits 80 and90, respectively. The charge stored on C1 and C2 is transferred to thetissue 70 through a plurality of electrode drivers 100-i forming adriver array 100 comprised of electrode drivers 100-1, 100-2, . . . ,100-i, . . . 100-n. The drivers act as a controlled energy transporterto allow stimulation of the tissue 70 in form of bursts of biphasic(anodic and cathodic) current pulses. Each driver can comprise, forexample, a constant current source or sink circuit, as later shown inFIG. 2. Each driver is connected to a respective stimulation electrode245-i (see also FIG. 2). The plurality of stimulation electrodes 245-iforms a stimulating electrode array 245 in direct contact with thetissue 70, as shown in FIG. 1-2. As also explained above, while thepresent embodiment makes reference to a retinal tissue, other types ofhuman or animal tissue can be envisaged by the person skilled in theart.

The present disclosure provides for monitoring and control ofexcessively low capacitor voltage. In particular, when charges aretransferred from capacitors C1, C2 to the tissue 70, the voltage on thecapacitors drops. A minimum value for this voltage is defined by aso-called compliance voltage. The compliance voltage is defined by theelectrode-tissue impedance at the interface 245-70 and the stimulatingcurrent 225 flowing between a driver 100-i and a respective electrode245-i. If the voltage on C1 and C2 falls too close to the compliancevoltage, the respective driver 100-i will not be able to maintain therequired amplitude of the stimulation current 255.

In order to prevent this from happening, a compliance monitor circuit110 is provided. Circuit 110 monitors occurrence of the above situationand notifies the external unit 20 through the back telemetry 160-180 tolower the stimulation current amplitude 255 by way of externalcontroller 700 and the transceiver 710 and forward data link 40, orincrease the capacitor voltage accordingly by way of the externalcontroller 700 and coil driver 720.

The implanted device 10 is continuously powered and controlled by theexternal unit 20 through the inductive power link 30 and the data link40, respectively. The external unit 20 comprises an external controller700, a coil driver 720 and a data transceiver 710. The externalcontroller 700 can include an information collector such as a camera inthe case of retinal prosthesis, a microphone in the case of cochlearprosthesis, or some other form of sensory devices such as pressure,position or touch sensors for various other neuronal-stimulationapplications. The external controller 700 can include a Digital SignalProcessing Unit or a similar operation processor to synthesize thesensed information from the sensors and the feedback information fromthe implant 10 and generate controls accordingly to command theimplanted device to deliver appropriate stimulation (amplitude andtiming) to the tissue through the data transceiver 710. The datatransceiver 710 ensures that the commands from the external controller700 are delivered to the implant 10 reliably and the feedback from theimplant 10 is received correctly. The data transceiver 710 communicateswith the implant 10 in predefined communication protocols through itsdata antenna 730. In the meantime, the coil driver 720 ensures thatadequate but not excessive power is delivered to the implant 10 for theintended stimulation intensity.

Also excessively high capacitor voltage is monitored, to avoid use ofunnecessary high power to deliver the same charge. In particular, shuntregulators 80 and 90 include a circuital arrangement to program thelevel of the nominal capacitor voltage to a required value, for furtherpower saving, as later discussed with reference to FIG. 4.

A power monitor circuit 120 is further provided, to monitor the chargingand draining conditions of the capacitors, so that the external unit 20can optimize the RF powering condition (see coil driver/monitor 720) andalso stop stimulation when the implant 10 cannot be adequately powered.The power monitor circuit 120 will be explained in greater detail inFIG. 7.

The implant 10 further comprises an implant controller 130 and animplant transceiver 140. Implant controller 130 comprises a maincontroller 150 and a back telemetry (BT) controller 160. Implanttransceiver 140 comprises a data receiver 170 and a back telemetry (BT)transmitter 180.

Implant controller 130 and implant transceiver 140 allow the stimulationcontrol and power control to operate on a system level. In other words,they allow the external unit 20 to appropriately and accurately controlthe implant powering and stimulation.

The data receiver 170 receives forward telemetry (FT) data from theexternal unit 20 through data link 40. FT data is decoded by the maincontroller 150 to control the output of the electrode drivers 100, therail voltages (as later shown in FIG. 4) and other operations, such asmonitoring of the electrodes and the device status for safety reasons orfor conducting system tests, and so on.

BT controller 160 collects and encodes implant information from thecompliance monitor 110 and the power monitor 120, such as implantpowering, stimulation, electrode condition and other safety informationof the implant, e.g. device failure, electrode failure, excessive powercondition etc. BT controller 160 sends the implant information back tothe external unit 20 through BT transmitter 180, so that the system canact on such information.

For example, the output of the power monitor 120 is fed to the BTcontroller 160, which will include this information in the backtelemetry stream. If the external unit 20, upon receiving the backtelemetry data, determines that the currents flowing through the shuntregulators 80, 90 are too high, it will adjust the input to the coildriver 720 to lower the level of power driving the coil by an amountthat is predetermined by the power control protocol. After the coilpower is lowered, the shunt regulator currents will decrease, thusforming a closed control loop.

A possible type of electrode driver 100-i to be used in the presentdisclosure is a constant current type electrode driver. Current typeelectrode drivers output current pulses whose timing, duration andamplitude can be controlled through input commands. See, for example, “ANeuro-Stimulus Chip With Telemetry Unit For Retinal Prosthetic Device”by Liu, W., Vichienchom, K., Clements, M., DeMarco, S. C., Hughes, C.,McGucken, E., Humayun, M. S., De Juan, E., Weiland, J. D., Greenberg,R., Solid-State Circuits, IEEE Journal of, Volume 35, Issue 10, October2000, pages 1487-1497.

In this respect, FIG. 2 shows a more detailed circuital diagram of oneof the electrode drivers 100-i of FIG. 1, the electrode driver being aconstant current electrode driver.

Electrode driver 100-i comprises a driver controller 190. The drivercontroller 190 takes commands from the ‘StimControls’ signals 195 comingfrom the main controller 150 and translates those signals into anodicand cathodic stimulation switching signals 200 and 210 that controlpulse duration and timing. Driver controller 190 also provides an outputcurrent 220 that defines the current amplitude of the stimulationphases. MOSFET transistors M16 and M18 convert current 220 into anodiccurrent 230. MOSFET transistors M18, M15, M11 and M12 convert current220 into cathodic current 240. MOSFET transistors M13, M14 and M17 actas output switches.

As already explained with reference to previously discussed FIG. 1-1through 1-3, electrode driver 100-i receives power from storagecapacitors C1 and C2 (see voltage signals V+, V− shown in FIG. 2).Storage capacitor C1 provides power for anodic pulses. Storage capacitorC2 provides power for cathodic pulses.

Similarly to FIG. 1-2, FIG. 2 shows a return electrode 250. The returnor common electrode 250 is connected to the junction Vce between C1 andC2. The cathodic stimulation currents 240, taken from C1, flow from thereturn electrode 250, through the tissue 70, to the V− rail of C1. Theanodic stimulation currents 230, taken from C2, flow from the V+ rail ofC2, through the tissue 70, to the return electrode 250. The electricalproperties of the electrode-tissue interface can be modeled to a certaindegree of accuracy by the simplified RC network shown in the shadedtissue circle.

FIG. 3 is a timing diagram showing the relationship between thestimulation timing, stimulation currents, surplus currents and voltageson capacitors C1 and C2.

During the cathodic stimulation phase 260, the cathodic stimulationswitching signal 210 (first graph on FIG. 3 from the top) is ON, thusturning ON both MOSFETs M13 and M14 (see also FIG. 2). On the otherhand, the anodic stimulation switching signal 220 (second graph on FIG.3 from the top) is OFF during that time interval, thus keeping theMOSFET M17 and the anodic current 230 OFF. Meanwhile, output currentsignal 255 (third graph on FIG. 3 from the top) carries the programmedcathodic amplitude control current 270 (FIG. 3), so that a current 240(FIG. 2) proportional to signal 220 (FIGS. 2 and 3) will flow from thereturn electrode 250 to V− through MOSFETs M14, M12 and the tissue 70(FIG. 2), causing a stimulation. When cathodic current 240 flows, chargeis drawn from C1, causing a dip ΔV 280 on the capacitor voltage VC1, asalso shown in the VC1 graph of FIG. 3.

The charge Q delivered during a stimulation phase isQ=Cathodic Current 240×Duration Switching Signal 210 ON

To the capacitor C1, a loss in charge Q means a drop in voltage ΔVΔV=Q/C1=(Cathodic Current 240×Duration Switching Signal 210 ON)/C1

If the capacitor C1 is being constantly charged with a current Icharge,the voltage on C1 at the end of the stimulation will beΔV=(Cathodic current 240−Icharge)×Duration Switching Signal 210 ON/C1

During the anodic stimulation phase 290, the anodic stimulationswitching signal 200 (second graph on FIG. 3 from the top) is ON, thusturning ON MOSFET M17, while stimulation switching signal 210 keeps thecathodic circuit OFF during that time interval. Output current signal255 carries the programmed anodic amplitude control current 300 (FIG.3), so that a current 230 (FIG. 2) proportional to signal 255 (FIGS. 2and 3) for the anodic phase will flow from V+ to the return electrode250 through MOSFETs M16 and M17 and the tissue 70 (FIG. 2), releasingthe charge collected on the stimulating electrode 245 during thecathodic phase 260 and achieving electrical chemistry balance. Whenanodic current 230 flows, charge is drawn from C2, causing a dip ΔV 310on the capacitor voltage VC2, as also shown in the VC2 graph of FIG. 3.Similarly to what discussed above, the voltage drop ΔV on capacitor C2isΔV=Q/C2=(Anodic Current 230×Duration Switching Signal 200 ON)/C2

It should be noted that when power is supplied to the drivers 100, thecharges on capacitors C1 and C2 are drawn only during the stimulationpulses (see time intervals 260 and 290 of FIG. 3), but are beinginjected by the power charging circuit 60 all the time. Therefore, thecharging current can be controlled to inject sufficient charges to thecapacitors as needed by the stimulation, to minimize the value ofsurplus currents IshL and IshH to achieve good power efficiency. Thecharging current is controlled through control of the power driving theexternal coil and the shunt regulator current value included in the backtelemetry data as the output of the control closes the power controlloop.

As also shown in FIGS. 1-2 and 2, the capacitor voltages V+, V− are thesupply rails to the electrode drivers 100-i. According to a furtherembodiment of the present disclosure, the capacitor voltages can belimited. Limitation of those voltages can be applied for two reasons. Afirst reason is that the electronic chip on which the implant 10 isoperated should operate under a specific voltage limit for safetyreasons. A second reason is that the power needed to inject a certainamount of charge in the capacitor is proportional to the capacitorvoltage itself. Therefore, limiting the capacitor voltage to a leveljust satisfying the need of electrode compliance voltages will alsoallow power to be saved.

As also explained with reference to FIGS. 1-1, 1-2 and 1-3, the voltagesof C1 and C2 are limited by shunt regulator circuits 80 and 90,respectively. In particular, shunt regulator circuits 80 and 90 canlimit the capacitor voltages to a preprogrammed value during charging.After the voltage on capacitors C1 and C2 reaches the safety voltagelimit, circuits 80 and 90 will bypass (shunt) the surplus chargingcurrent and the capacitor voltages will not rise further. The shuntingcircuits 80 and 90 may also comprise a current tap out, which provides aquantitative indication of the surplus current. Such indication can beused to estimate the power condition of the implant 10. Shuntingcircuits 80 and 90 can also comprise a rail control mechanism, whichallows the external unit 20 to set the capacitor voltage limit throughbinary control bits.

FIG. 4 shows in greater detail the internal structure shunt regulatorcircuits 80, 90, in accordance with a further embodiment of the presentdisclosure. The shunting circuit comprises a current sensor and acurrent sink. In the embodiment of FIG. 4, the current sensor iscomprised of a diode stack D1-D11 in series with a sampling resistor R1,and the current sinking circuit is comprised of an npn transistoramplifier Q1. During operation, the first terminal VH of the currentsensor is connected with either V+ or VM (see FIGS. 1-1, 1-2 and 1-3),while the second terminal VL of the current sensor is connected with VLeither VM or V− (see FIGS. 1-1, 1-2 and 1-3). Therefore, duringoperation, the two terminals of the current sensor are connected inparallel with the capacitors C1, C2. When the voltage across eachcapacitor reaches a value that is greater than the sum of the “knee”voltages of PN junction diodes D1-D11, the current through the diodesincreases rapidly, so as the voltage across sampling resistor R1 and thebase current of transistor Q1. The common emitter transistor Q1 willthen sink a larger current through its collector that is determined bythe current gain of the transistor. The sensitivity of the currentsensor is controlled by resistor R1. The person skilled in the art willunderstand that transistor Q1 can be replaced by any other arrangementsuitable for the same purpose. For example, if CMOS technology is used,the current gain of a single transistor may be limited, and Q1 could bereplaced with a Darlington pair and/or a transistor array to increasethe current gain for a better shunt effect. The person skilled in theart will also understand that the number of diodes D1-D11 can be anynumber, depending on the highest rail voltage specified.

The shunting circuit of FIG. 4 also comprises a transistor Q2 and acurrent mirror MOSFET circuit M1-M4. In particular, Q2 and M1-M4 providea current tap out Ishunt for quantitative measurement of the surpluscurrent flowing through the shunting circuit of FIG. 4. The outputsignal Ishunt is sent to power monitor 120, as already shown in FIGS.1-1, 1-2 and 1-3 and also later shown in FIG. 7.

The embodiment of FIG. 4 also provides for the presence of a railcontrol mechanism along connection RC between main controller 150 andshunting circuits 80, 90 (see FIGS. 1-1, 1-2 and 1-3). In particular,control of a rail voltage can be obtained by selectively shorting acertain number of diodes of the diode chain in the shunting circuitusing MOSFET switches (see switches S1-S3 in FIG. 4). As shown in FIG.4, the connection of the switches S1-S3 with the diodes D1-D11 providesa direct binary encoding, allowing the rail voltage limit to beconveniently programmed without the need of an encoding interface. Itshould be noted that the resolution of the rail voltage control is onediode voltage drop which is logarithmically proportional to the diodecurrent. In the embodiment shown in FIG. 4, only a small portion of thesurplus current will flow through the diode chain D1-D11. Therefore, thediode voltage drop is close to the turn-on voltage for the operatingdiodes, and the error caused by the ON resistance of the switches forshorted diodes is insignificant.

When the electrode voltage reaches the compliance limit due to increasedelectrode impedance, the output current 255 of the electrode driver cell(see, e.g., FIG. 2) will deliver less amount of current than what is setto be. This situation will affect the output accuracy of the implant 10and is also likely to result in unbalanced current pulses that are notfavorable to the electrodes 245 or the tissue 70. The electrodevoltages, however, are determined by the in vivo electrode-tissueimpedances which can vary greatly over time or among differentelectrodes, thus making any preset limit inefficient.

A further embodiment of the present disclosure addresses the above issueby providing a compliance monitoring circuit as already mentioned withreference to circuit 110 of FIG. 1-3, which circuit is shown in FIG. 5in greater detail. Although the compliance monitoring circuit 110 ofFIG. 1-3 has been shown separately from the driver array 100, suchcircuit, portions of which are represented in FIG. 5 with referencenumerals 320-i and 325, can be embedded into each current driver cell100-i. Therefore, according to a further embodiment of the presentdisclosure, there will be one compliance monitoring circuit per driver.Generally speaking, the compliance monitor 320-i/325 monitors thevoltage of electrode 245 to see if such voltage has reached thecompliance limit. When such limit is reached, the compliance monitorgenerates a signal CompAlert 340 (FIGS. 1-3 and 5) to alert the BTcontroller 160 and/or the external unit 20, so that a decision as towhether the rail voltage should be increased or the stimulation currentdecreased can be made.

Box 330-i of FIG. 5 represents a portion of driver cell 100-i of FIG. 2.Such portion 330-i comprises cathodic or sink current MOSFETs M11, M12,M13 and M14.

Generally speaking, ignoring the short channel effect of the MOSFET, thedrain output current of the electrode driver is set by a gate-sourcevoltage Vgs but not a drain-source voltage Vds as long as the outputMOSFET is in the saturation region, i.e., when the drain-source voltageVds is larger than the gate-source voltage Vgs subtracting the thresholdvoltage Vt of the device. The drain current falls off rapidly as thedrain-source voltage decreases further. This condition to hold the drainoutput current constant can be written as:Vds>=Vgs−Vt  (1)

In the electrode driver circuit, the above condition of drain-sourcevoltage Vds of the output MOSFETs, together with the power supply railvoltage Vp, defines the output compliance limit of the electrodestimulator. For example, if the power supply voltage is 10 volts, thethreshold voltage Vt is 1 volt, and the gate-source voltage Vgs is setat 1.5 volts, then the compliance limit can be defined asVcomp=Vp−(Vgs−Vt)=9.5 volts. The compliance monitor circuit according tothe present disclosure detects the condition of failing to meet (1),i.e., it detects the condition that fulfils:Vds<Vgs−Vt(2), or Vgs−Vds>Vt  (3)

The circuit shown in FIG. 5 is a direct realization of the condition ofequation (3). In a cathodic phase first stimulation protocol, thevoltage on the electrode 245 tends to shift towards a negative valuerelative to the return electrode 250 because of the capacitive componentin the electrode impedance. Therefore, the sink part (MOSFETS M11-M14)of the current driver is more susceptible to reaching the compliancelimit.

In the circuit of FIG. 5, MOSFET M12 is the output transistor that isresponsible for maintaining a stable current output. Vgs represents thegate-source voltage difference of M12, while Vds represents thedrain-source voltage difference of M12. The compliance monitor circuit320 monitors the difference Vgs−Vds of M12 by way of a subtractingcircuit 350 whose output is compared with the threshold voltage Vtreference 360 through comparator 370. The output of comparator 370drives an open drain nMOS transistor M21 whose output CC is fed to nodeVc, where the outputs of the compliance monitors of all drivers 100-1,100-2 . . . , 100-i, . . . 100-n are line OR-ed through MOSFET M100.

In normal operation, equation (3) is not fulfilled, and the output ofcomparator 370 is LOW, shutting off M21 so that its output CC does notaffect node 380. However, if the output voltage of the electrode 245reaches the compliance limit, equation (3) is satisfied and the outputof comparator 370 becomes HIGH, turning on M21, so that its output CCpulls down the voltage on node 380 to LOW. The voltage change on node380 will be sensed by a compliance logic circuit 390 to generate theCompAlert signal 340 discussed above.

FIG. 6 is a timing diagram for signal 210, output current 255, Vds,Vgs-Vds, CC and CompAlert, showing the behavior of those signals duringoperation of the compliance monitoring circuit discussed with referenceto FIG. 5.

According to a further embodiment, the present disclosure also providesfor power monitoring circuits 120 to monitor the charging and drainingconditions of the capacitors, as already explained with reference toFIGS. 1-1, 1-2 and 1-3. In particular, the power monitoring circuitsmonitor the surplus currents flowing through the shunting circuits 80,90 and the voltages on the capacitors C1, C2. Monitoring the surpluscurrent allows a flexible, quantitative control of the inductive powerloop, i.e. the loop including the coil driver, the coil pair, and thepower storage capacitors.

For example, the power level can be predefined (by way of a feedforwardmethod) or adaptive to the load requirement (by way of feedbackinformation). In addition, monitoring of the storage capacitor voltagesprevents the implant device 10 from operating with an insufficient powercondition that may compromise safety.

With reference to FIG. 4, applicants have already discussed how thesurplus current is tapped out as Ishunt through Q1 and M1-M4 in theshunting circuits 80, 90. FIG. 7 shows a current monitor 480 comprisingan analog-to-digital converter (ADC) 490 that digitizes the analogIshunt inputs IShH and IshL received from shunting circuits 80 and 90(see also FIG. 1-2). The converted digital output I_Level is sent to BTcontroller 160 and eventually reported to the external unit 20.

Reference can also be made to the flow chart shown in the followingFIGS. 8-1 and 8-2, where this current data is checked against a presetpower level (threshold) (see steps S4 and S20 of FIG. 8-1) and adecision is made as to whether increase or decrease the power output ofthe RF coil driver (see the left portion of FIG. 8-1).

Meanwhile, the voltages on C1 and C2 are also monitored by a capacitormonitoring circuit 500 comprising a subtracting circuit 510, comparators520, 530 and NOR logic 540. In normal power conditions, the outputCapLevel of the capacitor monitoring circuit is HIGH. If either one ofthe capacitor voltages VC1, VC2 falls below a preset threshold Vref(depending on the implementation of the circuit), the output CapLevelbecomes LOW. The LOW value of CapLevel indicates that implant power hasfallen below a critical low level required for safe operation. Uponreceipt of a CapLevel LOW signal, the external control will have to stopthe stimulating operation of the implant and raise the RF power outputlevel until CapLevel reaches again and stays at a HIGH value.

FIGS. 8-1 and 8-2 are a flow chart showing the system power controlflow. During start-up step S1 the PowerLevel limit (high and lowthreshold values of the digital output of ADC 490 in FIG. 7 to bechecked against) and the rail control bits (see FIG. 4) are set. Step S2checks the value of the CapLevel signal discussed in FIG. 7. If thevalue is HIGH, the implant is under normal operating conditions and theflow proceeds to step S3 where the value of the CompAlert signal 340 ofFIG. 5 is checked, to see whether the voltage on electrode 245 hasreached the compliance limit. In case such limit has not been reachedthe flow proceeds to step S4 to check whether the power (represented bythe surplus current from ADC 490) has exceeded the high threshold ofPowerLevel limit set in step S1. Should this not be the case, the flowproceeds to step S20 to check whether the power has fallen below the lowthreshold of the PowerLevel limit. If not (meaning that the implantpower is within the expected range), the flow goes back to step S2 forcontinuous monitoring. However, if the answer to the determination madein step S20 is affirmative, the flow first checks if the power drivingthe external coil (RF Power) has reached its maximum level (an externalunit specification) in a step S21. If it has not reached the maximumlevel, the coil drive is increased by an amount predefined in thecontrol protocol in a step S22 and the flow goes back to step S2 tocontinue a next round of monitoring of the implant power. If, on theother hand, the power driving the external coil has reached the maximumlevel so that no further increase of driving power is allowed, a lowpower alert of the implant is asserted in step S23.

On the other hand, if the answer to the question of step S4 is positive,which means that the implant power is excessive, the flow moves to stepS5 where it is checked if the power driving the external coil is at itsminimum level. If the power is not at its minimum, such power is loweredin step S6 (lower coil drive) and the flow goes back to step S2. On thecontrary, if the RF driving power level is already at its minimum,indicating that the PowerLevel overshoot of step S4 could be due tomalfunctioning of the external unit, safety measures are activated instep S7. Turning back to step S2, if the value of the CapLevel signal ofFIG. 7 is LOW, it indicates a critical low power level that could be dueto poor coupling of the coils, insufficient rail voltages, excessivestimulation current, or malfunction in the powering loop that includesthe external coil driver, the coils and the implant circuits. When thishappens, the flow first proceeds to step S8 where it is checked if thepower driving the external coil has been at its maximum. If thatcondition is satisfied, the flow proceeds to step S9 where the railvoltage is checked. If also that condition is satisfied (i.e. railvoltage at its maximum value), safety measures are activated in step S10to stop the stimulation operation. If, on the other hand, the railvoltage is not at its maximum value, such voltage is increased at stepS11 and the flow goes back to step S2. Similarly, if the coil drivingpower checked in step S8 is not at its maximum, such power is increasedin step S12 and the flow goes back to step S2. Turning to step S3, incase the compliance limit has been reached, the flow moves to step S13where it is checked if the rail control has set the rail voltage to itsmaximum value. If yes, the electrode impedance is checked at step S14and the maximum stimulation current is recalculated accordingly at stepS15. Once this has been done, the flow moves to step S4, alreadydiscussed above, to check the power level. Turning to step S13, if therail voltage is not at its maximum value, such value is increased atstep S16 and the flow moves to step S4 already discussed above.

While several illustrative embodiments of the invention have been shownand described in the above description, numerous variations andalternative embodiments will occur to those skilled in the art. Suchvariations and alternative embodiments are contemplated, and can be madewithout departing from the scope of the invention as defined in theappended claims.

What is claimed is:
 1. A compliance monitoring circuit to monitor andcontrol compliance voltage of an electrode suitable to contact a tissueof a human or animal body, the electrode being connected with anelectrode driver, comprising: a connection to the electrode providing anelectrode voltage (Ve); a predetermined reference voltage at a desiredvoltage level (Vr); a switching signal Vs; a predetermined thresholdvoltage (Vt) chosen to be a maximum voltage variation; an array ofMOSFET transistors including: a first MOSFET transistor having a sourceconnected to the electrode therefore having the voltage Ve, a gateconnected to the switching signal Vs and a drain having a voltage Vds; asecond MOSFET transistor having a source connected to the referencevoltage Vr, a gate connected to the switching signal Vs and, a drain; athird MOSFET transistor having a source connected to the predeterminedthreshold voltage Vt, a gate and a drain, wherein the gate of the thirdMOSFET is electrically coupled to the drain of the third MOSFET and thedrain of the second MOSFET therefore providing a voltage Vgs on the gateof the third MOSFET, the drain of the third MOSFET and the drain of thesecond MOSFET; and a fourth MOSFET transistor having a source connectedto the predetermined threshold voltage Vt, a gate electrically coupledto the gate of the third MOSFET therefore containing the voltage Vgs;and a drain that is electrically coupled to the drain of the firstMOSFET, therefore containing the drain voltage Vds; and a comparatorconfigured to compare the voltages Vgs, Vds and Vt, according to thecondition:Vgs−Vds>Vt and the compactor further configured to generate an alertsignal when such condition is met.
 2. The compliance monitoring circuitof claim 1, comprising: a subtractor, to subtract Vds from Vgs.
 3. Acompliance monitoring arrangement to monitor and control compliancevoltage of a plurality of electrodes adapted to contact a tissue of ahuman or animal body, each electrode being connected with a respectiveelectrode driver, the compliance monitoring arrangement comprising aplurality of compliance monitoring circuits in accordance with claim 1.4. The compliance monitoring arrangement of claim 3, wherein alertsignals of the compliance monitoring circuits are OR-ed together.
 5. Thecompliance monitoring circuit of claim 3, wherein each driver is aconstant current type electrode driver.
 6. The compliance monitoringarrangement of claim 1, further comprising communication means forcommunicating said alert signal.
 7. The compliance monitoring circuit ofclaim 1, further comprising a hermetic implantable housing around thecompliance monitoring circuit.
 8. The compliance monitoring circuit ofclaim 7, wherein the compliance monitoring circuit forms part of aneural stimulator.
 9. The compliance monitoring circuit of claim 7,wherein the compliance monitoring circuit forms part of a retinalimplant.
 10. The compliance monitoring circuit of claim 7, being coupledwith an external unit.
 11. The compliance monitoring circuit of claim 7,wherein coupling between the compliance monitoring circuit and theexternal unit is obtained through an inductive power link.
 12. Thecompliance monitoring circuit of claim 11, wherein the inductive powerlink comprises an implant coil.
 13. The compliance monitoring circuit ofclaim 11, further comprising a data link to allow exchange ofinformation between the compliance monitoring circuit and the externalunit.
 14. The compliance monitoring circuit of claim 1, wherein theelectrode is configured to stimulate the tissue by way of biphasiccurrent pulses.
 15. The system of claim 14, wherein the biphasic currentpulses comprise anodic pulses and cathodic pulses.
 16. A power controlsystem for an implant on a human or animal body, comprising: a chargingcircuit to provide power to deliver controlled stimulation currents to atissue of the human or animal body; a capacitive storage arrangementconnected with the charging circuit and charged by the charging circuit;an electrode driver array configured to transfer charges from thecapacitive storage arrangement to the tissue; an electrode arrayconnected with the driver array and the tissue; a connection to anelectrode of the electrode array providing an electrode voltage (Ve); apredetermined reference voltage at a desired voltage level (Vr); apredetermined threshold voltage (Vt) chosen to be a maximum voltagevariation; and a plurality of compliance monitoring circuits connectedto a respective electrode of the electrode array to monitor and controlcompliance voltage of each electrode of the electrode driver array,wherein each compliance monitoring circuit comprises an array of MOSFETtransistors including: a first MOSFET transistor having a sourceconnected to the respective electrode therefore having the voltage Ve, agate connected to the switching signal Vs and a drain having a voltageVds; a second MOSFET transistor having a source connected to thereference voltage Vr, a gate connected to the switching signal Vs and, adrain; a third MOSFET transistor having a source connected to thepredetermined threshold voltage Vt, a gate and a drain, wherein the gateof the third MOSFET is electrically coupled to the drain of the thirdMOSFET and the drain of the second MOSFET therefore providing a voltageVgs on the gate of the third MOSFET, the drain of the third MOSFET andthe drain of the second MOSFET; and a fourth MOSFET transistor having asource connected to the predetermined threshold voltage Vt, a gateelectrically coupled to the gate of the third MOSFET thereforecontaining the voltage Vgs; and a drain that is electrically coupled tothe drain of the first MOSFET, therefore containing the drain voltageVds; and a comparator configured to compare the voltages Vgs, Vds andVt, according to the condition:Vgs−Vds>Vt and the comparator further configured to generate an alertsignal when such condition is met.
 17. The system of claim 16, furthercomprising a shunting arrangement to limit voltage on the capacitivestorage arrangement.
 18. The system of claim 16, wherein the implant isa retinal implant.
 19. The system of claim 1, the system being coupledwith an external unit including means to communicate the alert signal tothe external unit.